1. Field of the Invention
The present invention relates to a driving circuit for driving elements such as exothermic resistors of a thermal head.
2. Description of the Prior Art
FIG. 1 is a circuit diagram showing a conventional driving circuit. In FIG. 1 are N exothermic resistors of a thermal head as driven elements, and Block 2 is a driving circuit for driving these N exothermic resistors marked 1 on the diagram, supplying each of them driving information individually.
In driving circuit 2, objects marked 3 are NAND gates which act as switching elements and are provided corresponding to the respective N exothermic resistors. Part 4 are shift registers for shifting the serial signal information to be the driving information for the exothermic resistors 1 inside it in synchronization with a clock signal, and 5 are latch circuits as holding means for holding the contents of the shift registers 4 according to a specified latch signal.
Part 6 is an input terminal for inputting the signal information serially to the shift registers 4 and 7 is a clock signal terminal from which the clock signal for the shift registers 4 is to be input. The points marked 8 are output terminals from which the signal information which is shifted inside the shift registers 4 is to be output, and 9 is a latch terminal from which a latch signal is to be input to the latch circuits 5. Part 10 is a strobe terminal to which a strobe signal to the NAND circuits 3 is to be input, 11 is a BEO terminal to which a negative enable output (hereinafter referred to as BEO) to the NAND circuits 3 is to be input, and 12 is a common electrode to which each of the exothermic resistors 1 is commonly connected.
FIG. 2 is a circuit diagram of a thermal head which is an example of an electronic part actually constituted based on the circuit shown in FIG. 1. In the figure point 13 are L pieces of integrated circuits (hereinafter referred to as ICs) which constitute the driving circuit 2, and each of them is constituted with N/L (the number of the exothermic resistors 1 N, divided by the number of IC's 13 marked L) NAND circuits, N/L stage shift registers and N/L latch circuits, and the IC's are connected in series with respective input terminals 6 and output terminals 8.
When a thermal head is, for example, of an A4 size of 8 bits per mm which is generally used in facsimile, etc., the total number of exothermic resistors 1 is 1723 and the total number of stages of the shift registers 4 is 1728, and when the constitutional unit of an IC 13 is assumed to be 64 bits, then it is found that 27 pieces of IC's 13 are connected. When split printing is to be performed, strobe terminals 10 and BEO terminals 11 of a number of systems corresponding to respective groups of IC's 13, composed of several pieces of IC's 13 have to be introduced.
Part 14 is a power supply terminal through which power is to be supplied to the IC's 13, and 15 is a terminal to be connected to the ground. The other points are similar to those shown in FIG. 1 so that the explanation of them will be omitted.
Next, the operation will be explained. The thermal head mentioned in the above is a main part of an apparatus of a thermosensible recording system, and recently the recording system has been widely utilized because of its simplicity. In the thermal head, numerous exothermic resistors are arranged on a substrate, and these resistors are selectively energized and the part of the printing paper which is pressed against an energized resistor is colored and printed. There are many kinds of driving systems and typical examples will be explained in the following.
The signal information for driving the exothermic resistors 1 is input to the shift registers 4 from the input terminal 6 in series, and it is shifted in order inside the shift registers 4 in synchronization with the clock signal given through the clock signal terminal 7. When the signal information for one line is input to the shift registers 4, a latch signal is input to the latch terminal 9 for the latch circuits 5, and data is transferred to the latch circuits 5 which hold the contents of the shift registers 4 according to the latch signal. The reason why the shift registers and the latch circuits are constituted as described in the above is to make it possible to transfer the signal information during the driving of exothermic resistors 1. The contents of the latch circuits 5 become the driving information for the exothermic resistors 1.
Each switching element 3 (NAND circuit) corresponding to each of the exothermic resistors 1 executes switching operation based on a strobe signal input to the strobe terminal 10, a BEO signal input to the BEO terminal 11 and the signal logic state of the driving information held in the latch circuit 5. In other words, the period of time during which the strobe signal is in an L level and the BEO signal is in an H level is the driving period of time for an exothermic resistor 1. The strobe signal and the BEO signal function respectively as decision signals for deciding the driving period time for exothermic resistors 1.
The reason why the operation logic of a strobe signal is made to be opposite to that of a BEO signal is to prevent an erroneous operation in a period of time of signal instability which can occur in a case of ON/OFF operation of the power supply in an apparatus using such a thermal head as mentioned in the above. The erroneous operation of a thermal head in the case of ON/OFF operation of the power supply can be prevented, for example, by making a BEO signal the detection signal of a voltage detector circuit of the power supply circuit.
FIG. 3 is a timing chart showing the operation of the driving circuit shown in FIG. 1. In the figure, COMMON is a voltage to be input to the common electrode 12, and may be 12V, 24V, etc., for example. DATA is signal information to be input to the input terminal 6, CLOCK is a clock signal to be input to the clock terminal 7, LATCH is a latch signal to be input to the latch terminal 9, BEO is a BEO signal to be input to the BEO terminal 11 and STROBE is a strobe signal to be input to the strobe terminal 10. A thermal head records an image on thermosensible paper by selectively energizing exothermic resistors 1, and the energizing time is adjusted depending on the ambient temperature or the temperature of a thermal head itself to upgrade image quality. Normally, the energizing time is controlled to be long when the temperature is low and to be short when the temperature is high. There is also a control method in which the temperature of a thermal head itself is raised by making a current flow in all exothermic resistors 1, the current which is not so large as to record an image on a thermosensible paper in a waiting time for preventing degradation of image quality caused by an insufficient energizing period of time in a low ambient temperature.
An example of a timing chart of an apparatus having the control method as mentioned in the above is shown in FIG. 4. As shown in the figure, an exothermic resistor 1 is energized in the period of time obtained by adding a period of time of "data-on by the driving information" to a period of time of "preheating by the signal information" for all black data. In this case, considering a countermeasure against electromagnetic interference (hereinafter referred to as EMI), it is desirable to make the transmission velocity of a clock signal lower than 500 kHz.
Since a conventional driving circuit is constituted as described in the above, there are problems as shown in the following: it is necessary to transmit signal information for preheating when the ambient temperature is low, and a printing velocity is decided by the transmission velocity of a clock signal. Also, degradation of picture quality is caused by the increase in heat accumulated by energizing the adjacent exothermic resistors 1 for preheating, and picture quality is degraded by heat conductance to adjacent exothermic resistors 1 when a continuous exothermic resistor 1 is formed with a thick film in a belt shape.